The present invention is generally directed to a chemical reaction supported by a catalyst, and more specifically relates to a method of converting carbon monoxide to carbon dioxide in the presence of hydrogen with reduced methanation.
The elimination of, or reduction in, carbon monoxide from a gas stream containing carbon monoxide, hydrogen and water vapor without significant consumption of hydrogen can be of value in numerous situations where the hydrogen will subsequently be used. One such example is in a fuel cell where hydrogen provides the fuel but carbon monoxide is a poison.
There are multiple fuel cell designs of which one is a Proton Exchange Membrane fuel cell (PEMFC). While the present invention finds utility with respect to PEMFC""s, and such use is described herein, the present invention should not be considered to be limited in such regard. The present invention also finds utility in other applications where reduced methanation is desirable.
A PEMFC includes an anode side and a cathode side separated by an electrolyte that acts as a membrane. On the anode side, hydrogen is introduced and each hydrogen molecule is split into two positively charged hydrogen ions. Simultaneously on the cathode side, oxygen molecules are introduced and each oxygen molecule is split into two negatively charged oxygen ions. The electrolyte is treated to conduct only positively charged ions, thus the negatively charged oxygen ions attract the positively charged hydrogen ions pulling them through the electrolyte. The electrons released in the splitting of the hydrogen molecule are conducted through an anode on the anode side to an external circuit to a cathode on the cathode side where the hydrogen ions and oxygen ions combine to form water.
Hydrogen is the fuel used to operate a fuel cell and must be generated, e.g. concentrated or released from a molecule containing hydrogen, as hydrogen is not available in a natural form suitable for use as a fuel. One source of hydrogen is fossil fuel, such as gasoline, that is reformed to release the hydrogen contained therein. Gasoline is particularly desirable as a hydrogen source when the fuel cell is to be used as a power plant in a non-stationary item such as an automobile. A problem with obtaining hydrogen from gasoline, however, is that in the reformation process hydrogen is generated in combination with other gases such as carbon monoxide.
Carbon monoxide contaminates the membrane within a PEMFC potentially rendering the fuel cell less efficient or inoperative. Therefore, carbon monoxide must be removed from, or substantially reduced in, the gas stream containing the hydrogen prior to the gas stream being introduced into the PEMFC.
One method of reducing the carbon monoxide in a gas stream containing carbon monoxide (CO), hydrogen (H2) and water vapor (H2O) is to convert it to carbon dioxide (CO2) and hydrogen using a catalyst (employing the water gas shift reaction CO+H2Oxe2x95x90CO2+H2). To accomplish this, low operational catalyst temperatures are preferred, since at high temperatures additional carbon dioxide can result in the production of carbon monoxide. High operational catalyst temperatures, on the other hand, are problematic in that the hydrogen tends to combine with either the carbon monoxide, or carbon dioxide, if present, to form methane (CH4) and water in a process referred to as methanation (CO+3H2xe2x95x90CH4+H2O, or CO2+4H2xe2x95x90CH4+2H2O). For each molecule of methane formed by methanation, the available hydrogen for the fuel cell is reduced. It is, therefore, desirable to employ a method that reduces the concentration of carbon monoxide without simultaneously reducing the hydrogen present, or at least minimizing the consumption of hydrogen by the methanation process.
Based on the foregoing, it is an objective of the present invention to develop a method for the removal of carbon monoxide from a gas stream comprising carbon monoxide, hydrogen and water vapor wherein the consumption of hydrogen therein is minimized thereby resulting in a gas stream having a higher concentration of hydrogen than would otherwise be obtained.
The present invention in one aspect is a method for the conversion of carbon monoxide to carbon dioxide in the presence of hydrogen with reduced methanation using a catalytic reactor having at least one channel defined at least in part by a catalytic surface. The catalytic surface is suitable for supporting a water gas shift reaction. At least a portion of the gas stream, which comprises carbon monoxide, hydrogen and water vapor, wherein the hydrogen is greater than or equal to five (5) times the carbon monoxide in terms of percent mole concentration, is then passed through the at least one channel at a flow rate such that a boundary layer formed over the catalytic surface has a thickness less than that of a fully developed boundary layer.
The gas stream can have other constituents, such as oxygen and carbon dioxide. However if oxygen is present, it should be minimized as it will combine with the hydrogen to form water. The consumption of hydrogen by oxygen to form water is equally undesired as is the consumption of hydrogen by carbon to form methane, where it is desired that the output gas stream contain as much hydrogen as possible. The molar ratio of oxygen should be less than 5:1 to that of carbon monoxide to minimize hydrogen consumption.
If carbon dioxide is present in the entering gas stream, the surface temperature of the catalytic surface should be maintained below about 450 degrees C. If the surface temperature exceeds about 450 degrees C., the carbon dioxide will combine with the hydrogen to form methane. Preferably, the concentration of carbon dioxide should not exceed a maximum amount, e.g. 25% at about 450 degrees C., which includes carbon dioxide created from the carbon monoxide.
The molar ratio of carbon monoxide to water within the gas stream is based on reaction stoichiometry. If it is the intent to convert all the carbon monoxide (CO) to hydrogen (H2), there needs to be at least an equimolar concentration of water to carbon monoxide. Therefore, it is preferred that the water to carbon monoxide ratio be greater than 1.0.
The invention relies on a controlling the thickness of the boundary layer within the fluid in the channel in the area of the catalyst such that the thickness of the boundary layer is less than the thickness of a fully-developed boundary layer. More specifically, a boundary layer is a region within a fluid flowing near a surface where the velocity of the fluid is less than the main bulk flow velocity. A boundary layer results from the viscous effects of the surface of the channel. These viscous effects are reduced the further the fluid is away from the surface, thus there is a velocity gradient within the fluid.
In a smooth channel of infinite length, the boundary layer begins at the entrance to the channel and increases to a fully-developed boundary layer at some penetration distance from the entrance. The penetration distance required to achieve a fully developed boundary layer is dependent upon the entering velocity and viscosity of the fluid. All boundary layers have a thickness, with the maximum thickness occurring when the boundary layer is fully-developed.
The thickness of the boundary layer can be maintained below the maximum thickness by assuring that the length of the channel is shorter than the penetration distance required to achieve a fully developed boundary layer. Alternatively, the thickness of the boundary layer can also be maintained below the maximum thickness in a channel having a length greater than the penetration distance by the use of flow disruption devices such as trip strips.
Based on flat plate geometry the velocity profile can be calculated using the equation:       V    x    =            U      ∞        ⁢          erf      ⁡              (                              1            2                    ⁢          z          ⁢                                                    U                ∞                            xv                                      )            
where:
Vx=the velocity in the channel in the x-direction
U∞=the free stream velocity
z=distance from the surface (perpendicular)
x=the penetration into the channel
v=viscosity
As gases tend to have generally equal viscosity""s, a fully developed boundary layer has a maximum thickness of about 0.03 inches for a given bulk velocity, temperature, pressure and viscosity. As those skilled in fluid mechanics will appreciate, as the viscosity of the fluid increases the maximum thickness for a fully developed boundary layer decreases.
The catalyst comprises at least one platinum group metal, which is defined as a group of transition metals that includes osmium, rhodium, iridium, palladium, and platinum, or gold. The invention should not be considered limited to the elemental forms of the platinum group metals, and gold as other forms such as molecules containing the platinum group metal could be used, such as oxides. The catalyst may be mixed with, or supported on, other substances such as alumina in any phase, or silica. The catalyst may also be stabilized, such as by the use of lanthanum.